I have bought a S.K. Grimes modified Graflex XL and want to put a 6x9 roll film back on it. Apparently some do fit and others don't. Can anyone tell me what brands are suitable for this camera, and what brands have trouble maintaining film flatness. Any help will be greatly appreciated.

From what I know of the "XL," it has a 2¼x3¼ "Graflok" back, and so will accept all the Graflex "Graphic" 2¼x3¼ 120, 220 and 70mm. rollfilm holders.

In general, there are three successive generations of 120 rollholders. The first offered knob wind, the second replaced that with lever wind, and the third retained lever advance while incorporating some changes.

One of those changes seems to have been designed to improve film flatness; you will see references on this site to "pins" added to accomplish this. I have used only the first-generation knob wind type, and I never had any problems with film flatness.

I don't know whether the rollfilm holders made for Linhof and Mamiya 2¼x3¼ press cameras deviated from the "Graflok" design. I do know they seem to be a good deal more expensive than the Graflex ones, and I am not sure what it is that you get for the extra money!

Information based on the assumption that standard Graphic roll holders fit the camera, which I believe is the case: Graphic "22" - knob wind, comes in 6X6 and 2X3 versions. Film flatness was an issue with these.
Graphic "23" - knob wind, comes in 6X6, 2X3 and maybe 6X7 (?). Some have pin rollers and some don't. Flatness issue for those without.
Graflex RH series holders - Lever wind. Comes in 6X6, 6X7 and 6X9. Available for 120 and 220 film. Pin rollers keep the film flatter. Model designations are by number of frames, RH-8, RH-10, RH- 12. Models for 220 film, the numbers a doubled.
The Mamiya RB-67 holders will fit any camera that will accept the above and come in 6X7 and 6X4.5 sizes. Some have a dark slide interlock that must be defeated in order to use then on Graflex cameras.